Alternators need time to replace battery charge. A weaker alternator will need longer than a stronger alternator.
Your driving conditions may not allow enough time, or the alternator is too weak there may not be enough power. A weak alternator will mean that the battery is drained down a little bit at a time.
Pretend the battery is your wallet, money is current and the alternator is your paycheck. If your paycheck isn't big enough, or doesn't come often enough, you'll go into deficit spending and eventually you're broke!
A long time ago, when I was a young'un,
: cars used dc generators. With the dawning of the solid-state age, diodes became available and alternating current technology could be used. "Alternating current generator" was a mouthful, so they shortened it to "alternator."
Automotive alternators generate more power for their weight than dc generators. All generators have brushes which wear down and need to be replaced at some point. Most alternators do not.
Diodes convert alternating current to direct current by blocking current in one direction. Clever winding and diode connection puts all of these one-way currents together and then out into the electrical system. For single phase, a "bridge" of 4 diodes is used. Most alternators are 3-phase use 6 diodes.
A shorted diode cannot block current, will not rectify ac into dc and will let current from the battery flow into the an alternator winding, draining the battery. In this case the problem appears in a short time
Hope this helps, or even makes sense! Good luck and good hunting.